Miniaturization of electronic devices has required innovation in the methods and materials used to fabricate smaller components. Electroplated metals can be fabricated, in a process called electroforming such that, at sufficient metal layers thicknesses the metal layers have substantial mechanical properties and may be used as structural members. Nickel is a common plated metal and alloys of nickel have been plated. Nickel is also a high temperature capable material with some ductility, thus it is a good candidate for mechanical structures. Additionally, nickel is electrically conductive, making it suitable for electronic applications.
As a pure metal, nickel is insufficient to meet the needs of some electroforming processes. The nickel plating can be alloyed with other metals to improve its strength, cost, ductility and thermal stability. Cobalt can be readily alloyed with nickel in the electroplating process. Cobalt levels as high as 60% by weight have been reported. Cobalt is a solid solution strengthener in a nickel cobalt alloy in which nickel is the base element. The alloy retains the face-centered cubic (FCC) crystal structure of the nickel alloy with some cobalt atoms substitutionally replacing nickel atoms in the FCC nickel lattice. Cobalt and nickel form a single phase solid solution alloy across substantially their complete composition range. In this single phase solid solution, some of the nickel atoms are replaced by cobalt atoms on the crystal lattice. The substitution of cobalt atoms for nickel atoms, which results in some lattice distortion with some strengthening of the alloy, acts to impede dislocation motion in the lattice and hence increase the yield strength and hardness of the metal. Cobalt additions can have other impacts as well, for example increases in magnetic permeability and modifying the curie temperature.
Sulfur is another common element resulting from electroplating solutions. Sulfur can be co-deposited in the nickel lattice during plating of nickel. Sources of sulfur can be tramp elements, such as sulfur-containing metallic impurities in the anode material, or in the form of intentional additives to the plating solution. Sodium saccharin or sodium naphthalene 1,3,6-trisulphonic acid are intentional additives used as stress relievers in nickel plating processes. However, sulfur levels from intentional additions to the plating solution must be controlled in applications that are exposed to elevated temperatures. At temperatures greater than about 200° C. (392° F.), nickel sulfide can form and preferentially precipitate at the grain boundaries (intergranular precipitation), which can embrittle the metal. Because of the problems associated with sulfur, is an unwanted element in the plated product, which is desirably eliminated or reduced to the maximum extent possible.
Other organic additives can be used to improve plating performance. For electroforming operations, the thickness of the plating deposit and the uniformity of that thickness can be important. Watson, in “Additions to Sulphamate Nickel Solutions,” Technical Publication Series No. 10053, Nickel Development Institute, 1989, described the use of 1,4 butyne diol as an additive in nickel plating to improve leveling of the nickel plating and throwing power. Boric acid is well known as a buffering agent and nickel bromide can be used to accelerate anode dissolution.
U.S. Pat. No. 6,150,186 discloses a process for plating a nickel-cobalt alloy, followed by a heat treatment process. One of the disclosed processes for depositing the alloy utilizes a plating bath the includes saccharin as an additive. The heat treating process at temperatures above about 200° C. (392° F.) transforms the as-plated structure to a structure having useful increases in materials properties as the coated material undergoes a transformation from a nanocrystalline, or amorphous, to a crystalline, or ordered, state. This process is called recrystallization and grain growth. Using the recommended heat treating processes produces an increase in crystal grain size as measured by x-ray diffraction. Endicott and Knapp, “Electrodepositon of Nickel-Cobalt Alloys: Operating Variables and Physical Properties of the Deposits,” PLATING, pp. 42-60, January 1966, showed that the microstructure can also convert from a layered structure to a more equiaxed structure as a result of heat treating nickel cobalt alloys.
While nickel based superalloys have often used rhenium as an alloying agent, these alloys use rhenium to retard other changes that may occur in the structure with time at temperature or for its refractory capabilities. These alloys cannot generally be manufacturing by electroplating and do not have the same composition as disclosed herein. Their chemical composition is a complex stew designed to maximize performance at elevated temperatures, usually above 538° C. (1000° F.). The complex composition also develops a complex microstructure that is suited to the environment that it will be used in, the microstructure developed by performing a complex heat treatment.
Nickel based superalloys have often used rhenium as an alloying agent to provide solution strengthening of the matrix phase or gamma phase of a two phase gamma-gamma prime (γ-γ′) structure at elevated temperatures for use in power generation applications in which the operating temperature is typically in the range of 1100-1200° C. (2000-2200° F.). However, these alloys use rhenium to retard other changes that may occur in the structure with time at these elevated temperature or for its refractory capabilities. These complex alloys are usually single crystal or directional in structure manufactured by casting techniques and remelting, followed by heat treatments to develop the single or directional crystal structure having complex precipitates. These complex alloys cannot generally be manufacturing by electroplating and do not have the same composition as disclosed here.
U.S. Pat. No. 6,899,926 discloses a plating process to make a rhenium alloy deposit which can contain nickel and cobalt. However, this alloy claims a rhenium content of 65% to 98% Re.
The state of the art to date has provided methods and materials to produce high temperature stable metals. These alloys can be used to electroform electromechanical structures of various shapes and sizes. In applications of interest now, the alloys must be used at continuous operating temperatures in excess of 150° C. (302° F.). The existing materials and processes provide insufficient performance in this temperature regime.
A critical mechanical property of interest is stress relaxation. Stress relaxation in metals is the reduction of tensile stress or applied force in a metallic member when deformed under a constant strain for a prolonged time. The relaxation can occur with time and is typically accelerated by increasing the storage temperature. This property can be measured in many ways. FIG. 1 shows an example of a stress relaxation plot for a heat treated nickel cobalt alloy exposed to a strain of 20% at 175° C. (347° F.) as measured in a dynamic mechanical analyzer (DMA). The alloy can support an initial load of 5 newtons, but after aging for 2500 minutes at 175° C. (347° F.), the alloy can only support 1.47 newtons. This is a relaxation of 70.6% of the original tensile strength of the material, alternatively stated as the material having only 29.4% stress remaining. A metallurgical phenomenon similar to stress relaxation is creep. The operating mechanisms are the same for creep and stress relaxation, but differ slightly in that in a creep application, the applied force or stress remains constant while the strain changes with time. For the purposes of this invention, stress relaxation and creep will be considered equivalent, if not identical, metallurgical mechanisms.